The efficient functioning of the hip joints is extremely important to the well being and mobility of the human body. Each hip joint is comprised by the upper portion of the femur which terminates in an offset bony neck surmounted by a ball-headed portion which rotates within a socket, known as the acetabulum, in the pelvis. Diseases such as rheumatoid- and osteo-arthritis can cause erosion of the cartilage lining of the acetabulum so that the ball of the femur and the hip bone rub together causing pain and further erosion. Bone erosion may cause the bones themselves to attempt to compensate for the erosion which may result in the bone becoming reshaped. This misshapen joint may cause pain and may eventually cease to function altogether.
Operations to replace the hip joint with an artificial implant are well-known and widely practised. Generally, the hip prosthesis will be formed of two components, namely: an acetabular cup component which lines the acetabulum; and a femoral, or stem, component which replaces the femoral head. During the surgical procedure for implanting the hip prosthesis the cartilage is removed from the acetabulum using a reamer such that it will fit the outer surface of the acetabular cup prosthesis. The acetabular cup prosthesis can then be inserted into place. In some arrangements, the acetabular cup component may be held in place by a tight fit with the bone. However, in other arrangements, additional fixing means such as screws or bone cement may be used. The use of additional fixing means help to provide stability in the early stages after the prosthesis has been inserted. In some prosthesis, the acetabular cup component may be coated on its external surface with a bone growth promoting substance which will assist the bone to grow and thereby assist the holding of the acetabular component in place.
The bone femoral head will be removed and the femur hollowed using reamers and rasps to accept the prosthesis. The stem portion will then be inserted into the femur. In some cases, a femoral component of this kind may be replaced with components for use in femoral head resurfacing or for use in thrust plate technology.
The correct positioning of the acetabular cup prosthesis is crucial to the efficient and long-term operation of the replacement hip joint. Just as the natural hip wears with time, the prosthesis will also wear with time. However, if the acetabular cup prosthesis is not correctly seated in the acetabulum, the wear rate of the prosthetic implant will be significantly higher than when the cup prosthesis is in the optimal position. Without wishing to be bound by any theory, it is believed that the optimum position is in the region of about 45° across the pelvis. It is believed that current techniques result in the cup being inserted at an error of about ±20°. It is further believed that at angles of about 50° or higher, excessive wear of the prosthesis will occur. It should also be noted that it is necessary for the correct working of the prosthesis that it is correctly aligned in both anteversion and vertical tilt.
In J. Bone Joint Surg. Am. 76: 677-688, 1994 “The Relationship Between the Design, Position, and Articular Wear of Acetabular Components Inserted Without Cement and the Development of Pelvic Osteolytes” Schmalzreid et al reported on a radiographic follow-up study of a series of 113 cementless metal on polyethylene resurfaced hips that had been implanted between 1983 and 1997. At a mean of 5.3 years after surgery, no acetabular component was radiographically loose, but osteolysis in the pelvis was noted in 17% of hips. Of these hips, there was a significant relationship between cup angles greater than 50° and osteolysis of the ilium. A trend was also noted in the hips for increasing wear with increasing cup angle. The authors of the report proposed that a decreased contact area generates higher stresses in the polyethylene component and that these stresses are responsible for the increased wear.
A retrospective review of 75 hip arthroplasties was discussed by Kennedy et al in J. Arthroplasty 13, No. 5, 530-534, 1998, “Effect of Acetabular Component Orientation on Recurrent Dislocation, Pelvic Osteolysis, Polyethylene Wear, and Component Migration”. This review investigated the effect of acetabular cup position on the wear of acetabular component in metal-on-polymer total hip replacements. The cementless acetabular component investigated had four anti-rotation ridges that the recommended surgical technique for the component specified that all four ridges should be in contact with the acetabular bone. In the first 38 patients, the cup was inserted in accordance with the recommended surgical technique with a mean angle of 61.9° in the frontal plane (i.e. 55° to 69°). In the subsequent 37 patients, the cups were inserted in a more horizontal position, with a mean angle of 49.7° (42° to 52°). The horizontal positioning required leaving one of the anti-rotation ridges on the acetabular component out of contact with the acetabular bone, i.e. an overhang of a few millimeters. Increased pelvic osteolysis, asymmetric wear and migration were found for the first group of patients. The increased wear, and subsequent migration and osteolysis, was attributed to the increased load per unit area, i.e. a smaller contact patch, generated by the vertically orientated cup, i.e. rim loading.
In J Bone Joint Surg Am 84-A, 1195-1200 “Cementless Hemispherical Porous-Coated Sockets Implanted with Press-fit Technique Without Screws; Average Ten-year Follow-up” Udomkiat P et al performed a 10-year follow up study of the Anatomic Porous Replacement cementless acetabular cup from Sulzer which had been used in metal-on-polymer low friction total hip replacements. This study considered 110 patients with replacements performed between 1988 and 1990. Volumetric and linear wear rates were significantly associated with younger patients and increased cup angle, i.e. more vertical orientation. No explanation is provided for the relationship between increased cup angle and increased wear, but the authors do mention that they now strive to keep abduction angles of the cup at ≤40′.
Brodner W et al, in “Cup Inclination and Serum Concentration of Cobalt and Chromium After Metal-on-Metal Total Hip Arthroplasty” J. Arthroplasty 19, 66-70, 2004, investigated whether cup inclination had an influence on the wear rate of metal-on-metal small diameter total hip replacements by measuring cobalt and chromium serum levels. Out of 330 patients, three groups of 20 were selected, based on their cup inclination angle. There was no significant difference between the cobalt levels (ρ=0.23) or the chromium levels (ρ=0.13) for the three groups, however three patients in the largest inclination angle group had notably higher metal ion levels. The authors concluded that metal cups with large inclination angles might be at risk of increased metal release. Reduced load transfer area was identified as the likely reason for increased wear with larger inclination angles.
A retrieval study “In Vivo Wear of Metal-on-Metal Hip Resurfacing Implants Depends Strongly on Cup Alignment” by Mortlock et al in 53rd Proceedings Orthopaedic Research Society Poster 1163, 2007 attempted to find a relationship between excessive wear and acetabular cup angle of 14 retrieved resurfacings. The results showed that there was a strong relationship between rim loaded implants, which were identified by wear patches extending to the edge of the cup, and high rates of wear. Cup inclination tended to be about 14° higher for the rim loaded implants, but cup inclination alone was not thought sufficient to define rim loading.
It will therefore be understood that correct acetabular cup orientation is therefore essential in total hip replacement/hip resurfacing. Various so-called “aerial” alignment guides have been suggested to assist the surgeon to correctly seat the acetabular cup prosthesis. These alignment guides are crude measuring devices that are subject to error depending on the position of the patient on the table.
The failure of these aerial guides to provide reliable positioning of the acetabular cup prosthesis was highlighted by Hassan et al in J. Arthroplasty 13, 80-84, 1998 “Accuracy of Intraoperative Assessment of Acetabular Prosthesis Placement”. In the described study four experienced surgeons performed a series of 50 total hip replacements. Using a conventional aerial guide, their goal was to position the cup between 30° and 50° vertical tilt and between 5° and 26 anteversion. Intraoperative assessment identified 47 of 50 cups being correctly positioned. However, radiographic measurement found only 20 cups within the defined zone. Even if a +2.5° error in radiograph measurements is assumed, only 22 cups were within the defined boundary. Malposition was more common in anteversion (18/50) than in the frontal plane (10/50).
It is therefore accepted that the position of the acetabular cup prosthesis influences wear rates for all types of hip arthroplasty. For metal/polymer articulations, this may be due to the reduced contact area when rim loading occurs. However, problems also occur with other arrangements and large diameter metal on metal bearings may be especially at risk due to increased stress caused by edge loading and possibly also the breakdown of fluid film lubrication. Cup placement is therefore an important aspect of the surgical procedure, especially for the large diameter metal on metal bearings. Surgical technique must emphasise the strongly negative effect of vertically positioned cups, but cups positioned too horizontally may also create problems in terms of impingement and range of motion. It is therefore desirable to provide a system which will reduce the variability in cup position.
Various solutions have been proposed to address the issue of surgical accuracy in cup positioning, the most costly of which is image guided surgery. Although this technique can improve accuracy if used properly, it is extremely expensive and may be beyond the financial resources of many orthopaedic units. There is therefore a need for a simple to use guide which is of low-cost.
A simpler solution was proposed by Echeverri et al in J. Arthroplasty 13, Vol 21 No 3, 80-84 2006, “Reliable Acetabular Cup Orientation with a New Gravity-Assisted Guidance System”. The proposed arrangement uses two fixed points on the pelvis, the hip joint centre and the anterior superior iliac spine, and two circular “bulls-eye” spirit levels. The first spirit level is fixed to a Schanz pin that is attached to the iliac crest bone, which requires a stab wound over the iliac crest, keeping the pelvis in lateral decubitus, and the second spirit level was attached to the shaft of the introducer/reamer, keeping the shaft in 45° abduction and 15° anteversion. By looking along an attached guide rod and lining it up with the anterior inferior iliac spine, the introducer was kept in the correct location. This arrangement is claimed to give impressive results in experimental testing when compared to the conventional “aerial” style alignment guide. However, the proposed arrangement suffers from various drawbacks in that it requires that the pelvis must be kept vertical throughout the reaming-introducing-impaction processes. In addition, the bulls-eye spirit levels can be difficult to centre.
U.S. Pat. No. 6,743,235 describes a modular instrument for use in positioning an acetabular cup prosthesis. The instrument comprises a hemispherical ball member which is adapted to cooperate with the acetabular prosthetic socket and an alignment shaft which is connected through an orientation pillar to a levelling apparatus. The instrument also has an intermediate shank, a distal handle and an impaction knob. The lower end of the orientation is coupled and anchored into the shank of the alignment shaft at an oblique angle of 135°. The upper end of the orientation pillar is provided with a levelling apparatus and the intermediate part of the orientation pillar is provided with a laser pen apparatus. When the acetabular cup is correctly positioned, the alignment shaft is abducted at an abduction angle of 45° such that the orientation pillar becomes perpendicular to the floor of the operating room or to the horizontal axis of the patient and the air bubble in the levelling apparatus migrates to the centre. The laser pen apparatus is located in a tray connected to the alignment shaft by a hinge mechanism. However, it is an independent parameter, adjustable as to the anteversion for the acetabular cup prosthesis. Once in position, the laser pen apparatus is switched on such that the beam projects across the acetabulum or pelvis while fine adjustments in the angel of anteversion are made.
In U.S. Pat. No. 6,743,235 it is suggested that the device should be angled at 45° to the vertical with respect to the earth, not to the pelvis as it is stated that if the pelvis is not in strict lateral decubitus, the cup will not be inserted at 45° to the pelvis. As the whole pelvis apart from the acetabulum is obscured by drapes, soft tissues or blood during surgery, keeping it in strict lateral decubitus during surgery is difficult.
In J. Arthroplasty 19:992-997, 2004, “Intraoperative Pelvic Motion on Total Hip Arthroplasty”, Asayama et al report that the pelvis can move up to 9° in the frontal plane, 31° in the horizontal plane and 18° in the sagittal plane during surgery. Such movement introduces serious inaccuracies into the cup angle position.
An alternative arrangement is described in U.S. Pat. No. 6,214,014. Here an apparatus is used to estimate the actual inclination of the acetabulum. A compensation is then made using a goniometer. A laser pointer is inserted into the end of the goniometer and a target can be marked on the wall of the operating theatre. The surgeon then uses this target to align the insertion tool for the acetabular prosthesis. The position can be rechecked after insertion by re-aiming at the target. Whilst the laser pointing device may be inserted into an aperture at the end of the insertion tool to check the position, the laser pointing device is not present during the impaction.
There is therefore still a need for a system which is simple to use and which will provide a reliable means to enable a surgeon to ensure that the acetabular cup prosthesis is correctly aligned. It is also desirable to provide an apparatus that is of low cost.